The present invention generally relates to composite materials and specifically to the protection of silicon carbide containing materials in hot, oxidizing environments such as ceramic matrix composites (CMC""s) in which at least one of the components of the CMC""s includes silicon carbide (SiC). Silicon carbide containing materials have not been used in hot oxidizing atmospheres such as the combustor or turbine portion gas turbine engines for various components because of problems with oxidation of the SiC.
SiC has a tendency to oxidize, the SiC being converted into silica (SiO2) and CO2 at elevated temperatures. Even when coatings are utilized for protection of the underlying SiC in hot, oxidizing environments, there is a tendency for oxygen to diffuse through the coating to oxidize the SiC to form carbon monoxide, CO, or carbon dioxide, CO2, and silica. This transformation also produces voids that exist in one of several places. The voids may appear in the external coating and/or in the interfacial silica scale formed by the transformation. The voids can also exist at the interfaces between the silica scale and the substrate. Of course the voids can also form tat the interface between the silica scale and the external coating. The voids are undesirable as thy decrease the environmental protection provided by any external coatings. Not only do the voids break down the SiC, which can adversely affect the mechanical properties of the CMC composite, which are designed around the mechanical properties of its system components, but also the voids can provide a path of least resistance that permits the continued inward diffusion of oxygen to promote further deterioration of the SiC composite. The voids can aggregate during the course of operation at high temperatures and can reduce the life of the external coating by promoting spallation of the applied coating.
Various systems are available for protection of carbon/carbon composites such as silicon carbide systems, from oxidation. One of these systems is a complex coating that includes an outer layer of a first coating comprising titanium diboride, colloidal silica and an intermetallic compound applied over a thin, primary coating of boronated silicon carbide as set forth in U.S. Pat. No. 5,536,574 to Carter. The invention improves the oxidation protection of a carbon/carbon composite wherein the secondary coating of diboride, colloidal silica and an intermetallic compound applied over the primary coating melts to form a crystalline glass ceramic coating. While the coatings improved oxidation protection to the substrates at temperatures of about 2400xc2x0 F., the temperature capabilities and times at temperature of the crystalline glass ceramic coating formed by melting at temperatures in the range of 2200-2400xc2x0 F., is limited by its melting temperature.
Other approaches such as set forth in U.S. Pat. No. 5,776,550 to Disam et al. (""550 Patent) and U.S. Pat. No. 5,741,596 to Skowronski et al. (""596 Patent) improve oxidation protection of metal substrates by utilizing variations of the glass/ceramic coatings formed of metallic silicides. The ""550 patent utilizes a boron-containing silicide to form a low melting oxide mixtures that have improved crack-healing properties as compared to pure SiO2. The ""596 Patent teaches the use of multi-layered coatings, the last of which is a thin layer of SiO2 applied by the sol-gel process.
Approaches for protecting CMC composites against oxidation are described in U.S. Pat. No. 5,246,736 to Goujard et al. This process utilizes a ternary Sixe2x80x94Bxe2x80x94C system to form a borosilicate glass and avoids the drawbacks encountered when there are superposed layers of precursors for boron-based glass and for silica-based glass.
Yet another approach is set forth in Thebault et al. in U.S. Pat. No. 5,622,751 which utilizes a coating of a mixture of non-oxide refractory ceramic, refractory ceramic and a polymer to form a commingled network.
What is needed is a system that can be readily applied to a substrate of a ceramic matrix composite material that forms a barrier to diffusion of oxygen that is both easy to apply to the substrate and adherent to the substrate. The diffusion barrier must be easy to apply and be capable of maintaining its resistance to oxygen diffusion, even at temperatures of up to 3000xc2x0 F. Ideally, the material should be also improve the adherence of thermal barrier coatings to the CMC substrate so that the component to which it is applied can be used in environments that experience even higher temperatures.
Improved adhesion of thermal barrier coatings to nonmetallic substrates is provided by applying a dense ceramic layer on the underlying nonmetallic substrate that includes at least one oxidizable component by appropriate methods that permits the formation of a dense ceramic coatings such as silica. The improved adhesion occurs because the application of the dense ceramic layer inhibits the penetration of oxygen by forming a diffusion barrier. This protection is required for articles such as gas turbine components designed to operate in high temperature, oxidizing atmospheres such as is found in the combustor portion of gas turbine engines and in sections of the engine downstream from the combustor portion. While silica has been used to protect substrates, it usually has been used in multi-layer systems or has been applied by application of SiC followed by thermal decomposition to yield silica and associated voids. The present invention applies silica by a process that deposits a relatively thick and dense layer of silica on the underlying CMC composite. The formation of the dense layer of silica avoids the problem of void formation associated with silica formation by thermal decomposition. Thus, the silica coating formed will not only provide protection of the underlying substrate from decomposition due to oxidation at elevated temperatures, but also will provide improved adhesion for coatings such as thermal barrier coatings applied over the component to increase its ability to perform at elevated temperatures.
An advantage of the present invention is that it forms a protective, diffusion barrier layer over the CMC composite that prevents the diffusion of oxygen through it. This prevents the deterioration of any materials present in the CMC that are subject to degradation by oxidation.
Another advantage of the present invention is directly related to the ability of the dense silica layer to inhibit form a diffusion layer. Since oxygen cannot penetrate the silica layer, the formation of voids at the interface or below the interface is prevented. As void formation is related to spalling of layers above the voids, particularly as the voids coalesce.
Another advantage is that the silica can form a strong bond with most non-metallic engineering materials that can be used as structural components in the hot portions of gas turbine engines. The typical thermal barrier coatings that overly these structural components to improve their thermal response also form are capable of forming good bonds with the dense silica.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.